Re: [6.2 regression][bisected]discard storm on idle since v6.1-rc8-59-g63a7cb130718 discard=async

Linux regression tracking (Thorsten Leemhuis)
2023-04-05T08:17:17Z

On 04.04.23 21:39, Boris Burkov wrote:
> On Tue, Apr 04, 2023 at 09:22:05PM +0200, David Sterba wrote:
>> On Tue, Apr 04, 2023 at 11:51:51AM -0700, Boris Burkov wrote:
>>> Our reasonable options, as I see them:
>>> - back to nodiscard, rely on periodic trims from the OS.
>>
>> We had that before and it's a fallback in case we can't fix it but still
>> the problem would persist for anyone enabling async discard so it's only
>> limiting the impact.
>>
>>> - leave low iops_limit, drives stay busy unexpectedly long, conclude that
>>> that's OK, and communicate the tuning/measurement options better.
>>
>> This does not sound user friendly, tuning should be possible but not
>> required by default. We already have enough other things that users need
>> to decide and in this case I don't know if there's enough information to
>> even make a good decision upfront.
>>
>>> - set a high iops_limit (e.g. 1000) drives will get back to idle faster.
>>> - change an unset iops_limit to mean truly unlimited async discard, set
>>> that as the default, and anyone who cares to meter it can set an
>>> iops_limit.
>>>
>>> The regression here is in drive idle time due to modest discard getting
>>> metered out over minutes rather than dealt with relatively quickly. So
>>> I would favor the unlimited async discard mode and will send a patch to
>>> that effect which we can discuss.
>>
>> Can we do options 3 and 4, i.e. set a high iops so that the batch gets
>> processed faster and (4) that there's the manual override to drop the
>> limit completely?
>
> Yup, I'm on it.
> [...]

Thx everyone for looking into this and the fruitful discussion
yesterday, much appreciated.

/me will now get back to watching this only from afar and leave things
to the experts

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
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